The Vaccine Testing Unit is involved in the development of new vaccines designed to protect against enterotoxigenic E. Coli diarrhea. Among these vaccines is a novel vaccine which stimulates antitoxid immunity after the vaccine is applied topically to the skin without injection. The vaccine has, in studies at Walter Reed, already been shown to stimulate high serum titers of antitoxin, but it is not known if the serological response will correlate with protection against disease. The purpose of the present study is to establish a volunteer model of diarrhea induced by pure LT toxin in order to determine the protective efficacy of the topical vaccine in future studies. In order to establish the model, low doses of LT will be given orally, and the dose will be increased in subsequent groups of volunteers until a dose is found which stimulates consistent, but not severe watery diarrhea in most of the volunteers.